This invention relates to binder-type carriers, more particularly, to binder-type carriers suitable for a developing method of electrostatic latent images, in which developers consisting of toners and magnetic carriers are carried to a developing region by a rotating developing sleeve with a magnet in the inside and the electrostatic latent images on the surface of a photosensitive member are developed in the developing region.
Conventionally, a magnetic brush of magnetic developers contacted with a photosensitive member which functions as a carrier of electrostatic latent images is formed by magnetic forces on the surface of a developing sleeve to develop the electrostatic latent images on the photosensitive member, in which mixtures of magnetic carriers such as iron particles of about 100-200 .mu.m in a mean particle size with insulating toners of about 10-20 .mu.m in a mean particle size were used as developers. The developers lead to problems such as white lines in developed images because an ear of the brush is hard due to a strong attractive force between the carrier particles and the carrier particles aggregate in the form of chains or fins on the surface of the developing sleeve. When the content of toner decreases by continuous use of the developers further problems occur such as the disorder of the electrostatic latent images, the deficit of the developed images and the adherence of the carriers to a part of the electrostatic latent images on the photosensitive member and the like. This occurs because the charge of the electrostatic latent images on the photosensitive member tends to discharge through the low specific resistant carriers of 10.sup.6 .OMEGA..cm or less. An additional defect is that the hard carriers adhered to the electrostatic latent images on the photosensitive member damage the surface of the photosensitive member when they are cleaned by, for example, a blade cleaner.
On the other hand, in order solve the above problems caused by the carriers consisting only of magnetic materials, the U.S. Pat. No. 4,284,702 made public binder-type carriers of 5-30 .mu.m in a mean particle size, in which magnetic fine materials are dispersed in such an insulating materials as resin, etc. and they have been put in practical use. The above binder-type carriers have an advantage that excellent images free from white lines are formed because of low magnetization of about 1000 gauss in the magnetic field generated in a general developing machine to be able to produce a soft ear. But, the binder-type carriers of this kind increase the cost of a developing machine by requiring a high torque-supplying motor as well as a weak point of heat of a developing sleeve in application thereof to recently required high-speed development.
A generally desired developing machine has a system in which a magnetic brush of a develooper formed by rotating a magnet inside the developing sleeve transports developer on a developing sleeve. But a change of magnetic polarity brought about by a rotating magnet, especially at low speed, results in surface irregularity, which tends to increase as the developing speed, i.e., the movement speed of an electrostatic latent image carrier, increases. It is necessary in order to prevent the above problem that a magnet is made rotate at as high a speed as possible. The rotating speed of the magnet, which is in general set within the range of 1000-2500 r.p.m., should be higher than the moving speed of the electrostatic latent image carrier. Thus, the higher the developing speed, the more the whirling electrical current increases. That results in heat of the developing sleeve to a higher temperature, besides the increment of a rotating driving load and the requirement of a high torque supplying motor. Further, some commercially available electrophotographic reproduction machines employ developing machines which are systematized not only with a high speed rotating magnet but also with an assistantly rotating developing sleeve. However, these machines cannot prevent the above mentioned problems.
A method comprising rotating only a developing sleeve with a magnet fixed (hereinafter referred to as "the developing sleeve rotating system") does not generate the problem caused by a rotating magnet as compared to the magnet rotating method. Accordingly, there may be a proposed attempt to apply binder type carriers for the magnet rotating system to the developing sleeve rotating system in order to solve the defect accompanied by the rotation of a magnet and simultaneously to provide good images free from white lines caused by the aggregation of magnetic materials. But, even if the binder type carriers are merely applied to the developing sleeve rotating system, the obstacle encountered in practical use is that a great number of carriers adhere to a part of the non-image on the surface of an electrostatic latent image carrier and so the above proposed attempt has not been put to practical use.
The higher the ratio of magnetic particles to binder resin in binder type carriers, the lower the electrical resistance of the carriers. Especially in the case of binder type carriers with small particle size, the hygroscopicity becomes higher enhancing the above problem.